1. Field of the Invention
The present invention relates to an image forming apparatus using a photoreceptor, and particularly to an image forming apparatus in which the photoreceptor is provided with a resistor, when executing a contact discharging and development of the photoreceptor.
2. Description of the Related Art
An image forming apparatus which forms an image by using a photoreceptor, such as copiers, first it charges the whole surface of the photoreceptor (primary charging) which is a previous step to form an electrostatic latent image on the charged surface of the photoreceptor (latent image formation). Secondly, the electrostatic latent image is developed so as to visualize the formed latent image (development). For the primary charging, a charging roller is frequently used for contact charging, instead of corona discharging to prevent from being produced ozone. In addition, a development roller is frequently used for development, where the development roller is in contact with the photoreceptor so as to supply charged toner. The charge is transferred from one-component or two-component developer to the photoreceptor.
For example, when the surface of the photoreceptor is negatively charged, and attempts executing a homopolar development, the voltages required for the drive of the charging and development come to minus several hundreds of volts. Therefore, each roller is provided with a resistor which is made of a material having a certain measure of resistance, such as rubber, for the purpose of avoiding short-circuit currents from flowing through when the above high voltages are applied from the respective metal shafts of the charging and development rollers. Accordingly, various analysis of the charging and development characteristics has been conducted by providing the resistor.
For example, Japanese Examined Patent Publication, No. 7-92617 discloses a charging model based on capacitance of a photoreceptor and resistance of a charging member. Japanese Unexamined Patent Publication, No. 3-87759 discloses a method of analyzing development characteristics by iterated calculations based on an electric field model. Japanese Unexamined Patent Publication, No. 1-230079 discloses a method of analyzing development characteristics of an insulated type development roller based on a capacitor model. Japanese Unexamined Patent Publication, No. 5-216263 discloses a technique for preventing a breakdown by an intermediate layer of a photoreceptor.
As mentioned above, the conventional charging roller is made of a high-resistance conductive elastic rubber, and is used for preventing the breakdown due to a pinhole in the photoreceptor. However, there have been problems in such rubber that it is difficult to suppress the variation of the resistances in each roller and the variation between the rollers. In addition, the resistance is greatly susceptible to the environment, in particular, humidity. Actually, the distribution of the resistance shows great range, e.g., about one-half to one order with respect to the resistance, in taking account of the variation in the resistance. Therefore, it is apt to need a roller of high performance and high accuracy, and therefore the cost would be increased.
Another image forming apparatus, a high-resistance layer is provided in the photoreceptor in order to prevent the breakdown in the photoreceptor, where the layer is treated with aluminum. However, in this case, an overcurrent due to a pinhole flows even if the breakdown does not occur, and thus causes the charging voltage drops due to activation of a power supply limiter. Nevertheless, no countermeasure is provided against the voltage drop. Therefore, this will lead to instability of charging voltage.
Further, the above problems are also found in the development rollers.